French Patent No. 1,149,159 discloses a process for removing the deep color caused by hydrogenation catalysts, from hydrogenated rubber polymers, by means of a mineral acid or of a monocarboxylic or polycarboxylic acid. The resulting polymers are transparent but with a yellowish tinge and have an improved transmittance achieved after a treatment which can take at least one hour on up to 36 hours. However, in the case of copolymers of resinous types, more particularly those disclosed in U.S. Pat. Nos. 3,639,517 and 4,091,053, the undesirable color is mainly due to the initiating agents used.
Not only for appearance sake, but also for uses in the field of foodstuff containers, it is very important to make this yellowish tinge disappear and to obtain completely colorless, transparent copolymers.
Methods are known for obtaining transparent copolymers by treating solutions of the copolymers with CO.sub.2 and water. However, this requires a distillation of the solvent to separate it from the water and the CO.sub.2 before it is recycled for polymerization, since otherwise these products would act as poisons to the initiator.
However, it must be remembered that in addition to transparency and coloration, the polymer must also maintain all its mechanical and shock-resisting properties, something which is not ensured by the aforementioned processes.
It is also known that an antioxidant additive must be added to these compositions in order to stabilize the butadiene phase during polymer transformation. In fact, the temperatures reached during this stage may cause a degradation of the polybutadiene phase. However, this kind of additive can clearly cause a coloring of the polymeric substance, so that it is desirable to reduce the quantity of antioxidant additive, and even more preferable to not have to add any.
European Patent No. 0084795 also suggests the treatment of polymers with carboxylic diacids. That type of process enables practically all the aforementioned decoloration conditions to be met, but without addressing the problem of the addition of an antioxidant additive.
European Patent No. 0309434 suggests that the copolymers should be treated with monocarboxylic acids containing chemically bonded sulphur. This process enables the amount of antioxidant additive that has to be added to be considerably reduced. Nevertheless, the process is only rarely used on an industrial scale, since the majority of these acids are solid at ambient temperature, having melting points higher than 100.degree. C., and are quasi-insoluble in the majority of usual solvents. This causes considerable difficulties when using the copolymers, the result being the clogging of the ducts or a rapid degradation of the copolymers.
It therefore is desirable to have a process for the treatment of vinyl aromatic/conjugated diene copolymers which ensures that the copolymers remain completely colorless and transparent, retain appropriate shock-resistant properties, are safe and desirable for packaging foodstuffs, are given an antioxidant function without the need for the addition of antioxidant additives, and lastly are readily usable.